Arc electrode of graphite with ball tip



Aug. 6, 1968 W. A. GORDON ARC ELECTRODE OF GRAPHITE WITH BALL TIP Filed March 2l,

QA s' United States Patent O 3,396,363 ARC ELECTRODE F GRAPHITE WITH BALL TIP William A. Gordon, Strongsville, Ghio, assignor to the United States of America as represented by the Administrator of the National Aeronautics and Space Administration Filed Mar. 21, 1966, Ser. Nn. 537,517 3 Ciaims. (Cl. 313-352) ABSTRACT 0F THE DlSCLOSURE An arc source for emission spectroscopic analyses in which the cathode electrode comprises a graphite post having a globular tantalurn tip.

The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates generally to apparatus for controlling an energy source for spectral analysis and more particularly to such apparatus for controlling a D.C. arc source for an emission spectrograph or spectrometer.

In making trace analyses by emission spectroscopy, a major source of diiiiculty has been the arc source used to excite the sample material. Commonly, the sample is held in a graphite crucible to provide the anode of the arc source, and a suitable potential is applied between this anode and the cathode of the arc source to produce an arc discharge for vaporizing the sample and for exciting the atomic spectra of the sample to produce light from which the trace analysis may be made by known techniques.

Ideally, the arc source in such equipment should have both high sensitivity and high precision. It has been recognized that a D.C. arc source is extremely sensiti-ve, but prior to the present invention there has been no entirely satisfactory way to achieve the desired precise control over the output energy of the D C. arc source. In practice this has meant that the analytical spectroscopist must have available to him a variety of diiierent arc sources, including a D C. arc source, a spark source, various combinations of the two, and high -voltage and low voltage A.C. arc sources. In the use of such arc sources the analytical spectroscopist must, in any speciiic instance, sacriiice precision for sensitivity, or vice versa.

The present invention is directed to an apparatus which avoids these diiculties and disadvantages by providing an improved arrangement for precisely controlling the output energy of an energy source for spectral analysis. As applied to emission spectroscopy, the present control regulates the output energy of a D.C. arc source7 so that the user may take advantage of the high sensitivity inherent in the D.C. arc source, without, however, sacrilicing the desired precision.

In accordance with the preferred embodiment of this invention, these objectives are achieved by controlling the vaporization rate of the sample, and therefore the output energy of the D.C. arc source, in accordance with a programmed signal, either a constant or a timedependent command signal, thereby making the output energy of the D.C. arc source independent of transient events which may take place in the arc source. The output energy of the D.C. arc source is compared with a programmed command signal and the difference between 3,3%,363 Patented Aug. 6, 1968 ICS them produces an error signal which regulates the direct current supplied to the arc source, so as to regulate the latters output energy in accordance with the programmed command signal.

Another aspect of the present invention is directed to stabilizing the arc of the D.C. arc source so that it is a steady, rather than a ickering, source of light, This is achieved by providing a novel and improved cathode structure for the arc source having a refractory metal tip which enables a more stable arc discharge to be maintained.

While the present invention is particularly applicable to the analysis of trace constituents in a sample by emission spectroscopy, it is also advantageous for use in determining major constituents of the sample, such as alloy constituents, by emission spectroscopy. Also, it may be used in mass spectrometry and in ato-mic absorption spectroscopy, if desired.

It is a principal object of this invention to provide a novel and improved apparatus for controlling the output energy of an energy source for spectral analysis.

Another object of this invention is to provide such an apparatus which controls the output energy of the energy source in accordance with a constant or timeprogrammed command signal, thereby eliminating the effects of transient events in the energy source.

Another object of this invention is to provide such an apparatus which is particularly useful for controlling the output energy of a DC. arc source for a spectrograph or spectrometer.

Another object of this invention is to provide such an apparatus having a novel and improved cathode construction for stabilizing the arc in a D.C. arc source for a spectrograph or spectrometer.

Further objects and advantages of this invention will be apparent from the following detailed description of a presently-preferred embodiment thereof which is shown in the accompanying drawing.

ln the drawing:

FIGURE, l is a schematic circuit diagram of a presently-preferred embodiment of the present apparatus; and

FIGURE 2 is an enlarged fragmentary longitudinal section of the cathode in the arc source in the FIG. l apparatus, in accordance with the preferred embodiment of this invention.

Referring first to FIG. l, an energy source in the form of a D.C. arc source 14B is associated with a direct reading spectrometer 11 of known construction, the details of which will be understood by those skilled in the art and therefore need not be described in detail.

The D.C. arc source lil includes a cathode 12 and an anode constituted by a graphite holder 13 and the sample 14 in the holder. The cathode and anode are in spaced confronting relationship with respect to `one another. A D.C. source for producing the arc across these electrodes is provided by an A.C. power supply source 1S and a phase-sensitive rectifier 16 connected between the output of this AmC. power supply source and the electrodes.

T he spectrometer 11 is provided with a photomultiplier tube 17 of known construction which senses the light output of a selected spectral line which is emitted by the arc produced by the source 10. That is, the light energy produced by atomic emission of a selected element in the arc is the input signal to this photomultiplier tube 17. The output signal from the photomultiplier tube 17 is an electrical signal on line 18 having a magnitude substantially proportional to the light energy produced by the arc. This output signal is applied to one input terminal of a differential amplier 19 of known design.

In accordance with the present invention a second input signal to this ditferential ampliiier is generated by a command signal generator 20 which may be either a timeprogrammed function generator or a constant signal generator of known design, which produces either a timedependent or constant command signal in accordance with a predetermined program. The time-dependent function generator may be a curve following programmer of any suitable design, or it may be a function potentiometer Of the general type shown at page 253 of the book, Electronic Analog Computers by Korn et al., published by McGraw-Hill Book Company, New York, 1952, for example.

In the differential amplifier 19 the output signal from the photomultiplier tube 17 is compared with the output signal from the command signal generator 20. The difference between the output signal from the photomultiplier tube 17 and the signal from the command signal generator 20 produces an error signal on the output line 21 from the differential amplifier 19. The sign and the magnitude of this error signal depend upon the respective magnitudes of the command signal and the output signal from the photomultiplier tube. This error signal is applied to the phase-sensitive rectifier 16 to control the magnitude of the latters output, which provides the current for the arc source 10 and therefore controls the rate at which the sample 14 is vaporized.

The arrangement is such that if the -output signal from the photomultiplier tube 17 has a higher amplitude than that of the signal from the command signal generator 26 (indicating that the output energy of the arc source 10, which is determined by the rate at which the sample 14 is being vaporized into the arc, is higher than that called for by the command signal), then the error signal applied to the phase-sensitive rectilier 16 will reduce the latters output, thereby reducing the direct current applied to the arc source 10 so as to reduce the latters output energy to the value called for by the command signal. Conversely, if the output signal from the photomultiplier tube 17 has a smaller amplitude than that of the signal from the command signal generator 20 (indicating that the output energy of the arc source is lower than what is called for by the command signal), then the error signal applied t the phase-sensitive rectifier 16 will increase the latters output, thereby increasing the direct current applied to the arc source so as to increase the latters output energy to the value called for by the command signal.

With this arrangement, therefore, the output energy of the arc source 10 is regulated in accordance with the signal of the command signal generator 20. This gives extremely precise control over the output energy of the arc source while at the same time retaining the advantages of the 4high sensitivity which is inherent in the D.C. arc source, as compared with other types of arc sources.

From the foregoing it will be evident that the present control arrangement achieves the desired objective of controlling the vaporization rate of the sample in a manner which enables the reproduction of that same programmed vaporization rate for subsequent samples. This is in contrast to previous proposals to provide a constant input current to the arc, which failed to achieve the desired objective -because the vaporization of the sample can itself change the arc current. Similarly, prior proposals to hold the arc current constant regardless of the rate at which vapors enter into the arc fail to achieve the objective of providing a reproducible vaporization rate of the sample.

While the preferred embodiment is aimed at controlling the vaporization rate of the analytical specimen, it may be desirous to control the excitation of elements subsequent to their entering the arc column. This may be accomplished by imposing control on the effective excitation temperature of the arc column. A further embodiment, therefore, of the control arrangement is where the reference radiation from the arc in the arc source 10 is comprised of two spectral lines, the ratio of which is a function of the effective excitation temperature of the arc. The electrical ratio of the two lines is the input signal on line 18 to the differential amplifier 19, and this signal is compared to the signal from the command signal generator 20. The matching of the ratioed reference signal to the command signal may then be accomplished by appropriate adjustments in arc current, gas pressure, gas composition or other parameters.

While the present control arrangement has been described with reference to the regulation of a D.C. arc source for an emission spectrograph or spectrometer, it may also be used to control the variable energy source in atomic absorption spectroscopy. For example, if the sample -being analyzed is in the form of a solution Ibeing aspirated into a flame which provides the excitation of the sample, the present control may be used to regulate the aspiration pressure or in some other way to control the rate of aspiration of the sample in accordance with a programmed control signal.

Another aspect of the present invention is directed to stabilizing the arc in the source 10. For trace analysis of a sample in a static inert atmosphere, such as argon, the free running D.C. arc has been found to be unstable, compared to its performance in air. In argon the arc produces a noticeable Ianode-flame, composed of plasma streaming toward the anode. This anode-dame tends to revolve erratioally about the axis of the electrode because of convection and buoyancy effects arising from the thermal gradients between the arc column and the surrounding atmosphere. This instability of the arc column leads to poor precision of measurement.

Referring to FIG, 2, in accordance with the present invention this diiculty is overcome by the provision of a novel cathode in the arc source 10 of FIG. 1. This cathode includes a graphite rod or post 22 having a tapered end 23 in which an axial recess 24 is drilled. In the arc source 10 l(FIG. 1) this tapered end of the post is disposed toward the anode. The cathode 'also has a refractory metal tip, preferably of tantalum, which presen-ts a generally cylindrical globular bead 25 extending across this end 4of the graphite rod and -an integral stem 26 which is received tightly in the recess 24 to secure the refractory metal tip to the graphite post. In one practical ernbodiment the refractory metal bead 25 is about 2 mm. in diameter.

When used in combination with a conventional anode arrangement in the D.C. arc source, it has been found that the cathode spot, when the are is maintained, is spread over substantially t'he entire surface of the bead 25, Consequently, the current density and the magnetic iields are reduced and the plasma streaming, or anodeflame, is greatly diminished or completely eliminated, thus providing a highly stabilized arc column.

When yused in emission spectrographic trace analysis, this novel cathode improves the reproducibility of line and background intensities and substantially eliminates arc wandering or fluctuations as .a source of err-or without reducing the sensitivity of measurement for either solid or liquid samples. The cathode, constructed as described, may be used for hundreds of trace analyses without the need for special treatment between exposures.

The arc is initiated preferably by using a spark igniter to ignite the graphite post 22. After about two seconds the refractory metal bead 2S will be sufficiently hot to thermally emit, at which time it takes over as the cathode.

While a presently-preferred embodiment of this invention has been described in detail with reference to the accompanying drawings, it is to ybe understood that various modifications, omissions and refinements which depart from the particular embodiment disclosed may be adopted without departing from the spirit and scope of the present invention.

I claim:

1. In a D.C. arc source for a spectral-measuring instrument having an anode Iand a cathode in spaced, confronting relationship to each other, the improvement which comprises a cathode structure comprising a graphite post and a refractory metal tip secured to said post and extending across the latters end which is disposed toward the anode, said post being of progressively reduced cross section towards said end, yand said refractory metal tip presenting a globular bead extending across said end of the post.

2. The cathode of claim 1 wherein an integral stern is formed on said globular bead, said post having -a recess in said end, said stem being disposed in said recess whereby said globular bead is -retained on said post.

3. The cathode structure of claim 1, wherein said refractory metal tip is of tantalum.

References Cited UNITED STATES PATENTS 5/1903 Dean 313-252 X 6/1937 Inman et al 313-252 X 12/1942 Toepfer 3113-252 X 12/ 1947 Rabinowitz 315-151 9/1958 Woll 315-199 FOREIGN PATENTS 10/ 1964 Great Britain.

DAVID J. GALVIN, Primary Examiner. 

